The assembly of active array antennas is very complex because of the number of components used in the array. Performance testing of the assembly antenna is conventionally conducted on the near field range in which a waveguide probe samples RF radiated power at discrete locations across the array. If a single radiating element malfunctions, the near field probe will typically be able to identify its location within the array. However, since the probe is also sampling RF energy radiated from all the remaining array elements at that same location within the array, through a phenomenon known as mutual coupling, the probe cannot provide more detailed information concerning the nature of the failure. Conventionally, to diagnose and correct the failure, it may be necessary to disassemble the antenna, retest all the components along the RF path to the identified failed element, correct the problem, and then reassemble the antenna. Another method of diagnosis is to incorporate built-in-test (BIT) circuitry within the antenna. BIT circuits are typically employed in antennas, but in limited quantities because of constraints on antenna size and weight.
It would therefore be desirable to be able to probe directly into the RF path of the failed radiator in the absence of mutual coupling effects and without disassembling the antenna. For metal flared notch radiators, one can surround the flared notch portion with a rectangular waveguide and sample RF energy from that. The problem with this approach is that such waveguide interconnection tends to be band limited and creates an impedance mismatch at any TEM transitions (such as stripline, microstrip, suspended stripline or coaxial line) within the radiator.